Just my opinion - but I would rather take the non-stop flight from YVR to YYZ aboard the A340s any day - instead of going to SEA/TAC airport just to save $ 200. But I am not the one flying.
If I were you I would take United to ORD (from YVR) and then ORD – YYZ. You earn full Aeroplan points plus have the possibility of being upgraded.
From past experience upgrading on United is much easier then with Air Canada / Aeroplan.
Have a nice trip.
JP